This invention relates to a process for drying green wood veneer sheets and to apparatus for carrying out the wood drying process.
Veneer is conventionally dried after being cut from a log by passing the green wood sheets on conveyor means through a large, elongated housing within which air heated by steam or gas burners is circulated so as to contact the top and bottom surfaces of the sheets and thereby remove moisture to the extent desired as well as small amounts of volatile organic materials which are present. In recent years, the process and apparatus have been modified to obtain more efficient drying by directing the hot air against the surfaces of the veneer as multiple small jets. Such modified apparatus is shown in Morris, U.S. Pat. Nos. 3,314,164; 3,334,421; and 3,418,727.
Hot air drying of veneer in general has the disadvantage that resin, pitch, and other organic materials volatilized from the wood during the drying process turn the exhaust air from the vent stack into a characteristic plume of blue haze. Even from an operation of moderate size, the volume of haze thus generated is not only unsightly, but is also a considerable source of highly undesirable air pollutants as well as a waste of potentially valuable organic substances. The exhaust haze can be reduced by use of a water scrubber although this treatment is not entirely effective and a particular problem is the deposition on scrubber wall surfaces of tenacious coatings of gummy and varnish-like solids.
Wood has been dried by application of hot solvent vapors, as shown by Hudson, U.S. Pat. Nos. 2,273,039 and 2,435,219, for example. These processes are designed for lumber, timbers, and poles and are carried out in sealed chambers, sometimes pressurized and sometimes at reduced pressure. The solvents employed are limited to those having a boiling point above 100.degree. C. Such methods are not adapted to drying veneer which is commercially done in line on a continuous or semicontinuous basis.